Molecular markers reveal that population structure of the human pathogen Candida albicans exhibits both clonality and recombination.

The life history of Candida albicans presents an enigma: this species is thought to be exclusively asexual, yet strains show extensive phenotypic variation. To address the population genetics of C. albicans, we developed a genetic typing method for codominant single-locus markers by screening randomly amplified DNA for single-strand conformation polymorphisms. DNA fragments amplified by arbitrary primers were initially screened for single-strand conformation polymorphisms and later sequenced using locus-specific primers. A total of 12 single base mutations and insertions were detected from six out of eight PCR fragments. Patterns of sequence-level polymorphism observed for individual strains detected considerable heterozygosity at the DNA sequence level, supporting the view that most C. albicans strains are diploid. Population genetic analyses of 52 natural isolates from Duke University Medical Center provide evidence for both clonality and recombination in C. albicans. Evidence for clonality is supported by the presence of several overrepresented genotypes, as well as by deviation of genotypic frequencies from random (Hardy-Weinberg) expectations. However, tests for nonrandom association of alleles across loci reveal less evidence for linkage disequilibrium than expected for strictly clonal populations. Although C. albicans populations are primarily clonal, evidence for recombination suggests that sexual reproduction or some other form of genetic exchange occurs in this species. IMAGES:

白色念珠菌（Candida albicans）的生活史一直是一个未解之谜：该物种曾被认为仅能进行无性繁殖，但其菌株却表现出广泛的表型变异。为解析白色念珠菌的群体遗传学特征，我们建立了一种针对共显性单基因座标记的基因分型方法：通过对随机扩增DNA进行单链构象多态性（single-strand conformation polymorphisms）筛选实现。研究首先对任意引物扩增得到的DNA片段进行单链构象多态性筛选，后续再通过基因座特异性引物对目标片段进行测序。最终在8个PCR扩增片段中的6个里，共检测到12处单碱基突变与插入位点。对单个菌株的序列水平多态性模式分析显示，其DNA序列水平存在显著的杂合性，这一结果支持“多数白色念珠菌菌株为二倍体”的观点。对杜克大学医学中心提供的52株自然分离菌株开展的群体遗传分析，为白色念珠菌同时存在克隆繁殖与重组现象提供了证据。克隆繁殖的证据主要包括两类：一是存在多种过度代表的基因型，二是基因型频率偏离随机交配（哈迪-温伯格，Hardy-Weinberg）平衡的预期值。然而，对位点间等位基因非随机关联的检测结果显示，其连锁不平衡程度低于严格克隆繁殖种群的预期水平。尽管白色念珠菌的种群以克隆繁殖为主，但重组现象的相关证据表明，该物种可能存在有性生殖或其他形式的遗传交换。IMAGES: